fix(git): print valid JSON/YAML output for list cmds

[rubenhoenle]

Description

relates to STACKITCLI-265 / #893

Testing

1. Test flavor list command output
   • stackit git flavor list -> Expected output: Table showing the available STACKIT git flavors
   • stackit git flavor list --output-format json -> should output valid JSON
   • stackit git flavor list --output-format yaml -> should output valid YAML
2. With no STACKIT git instance present in your project: Verify output of instance list command
   • stackit git instance list -> Expected output: No instances found for project "xxx"
   • stackit git instance list --output-format json -> should output valid JSON
   • stackit git instance list --output-format yaml -> should output valid YAML
3. Create a STACKIT git instance in your project: stackit git instance create --name my-new-instance --flavor git-100
4. With a STACKIT git instance present in your project: Verify output of instance list command again
   • stackit git instance list -> Expected output: Table showing the available STACKIT git instances of your project
   • stackit git instance list --output-format json -> should output valid JSON
   • stackit git instance list --output-format yaml -> should output valid YAML
5. Cleanup - delete the STACKIT git instance: stackit git instance delete xxx

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/git/flavor/list	56.52% (+6.52%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/git/instance/create	50.00% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/git/instance/list	56.52% (+6.52%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/git/flavor/list/list.go	56.52% (+6.52%)	46	26 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/git/instance/create/create.go	50.00% (ø)	52	26	26
github.com/stackitcloud/stackit-cli/internal/cmd/git/instance/list/list.go	56.52% (+6.52%)	46	26 (+3)	20 (-3)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/stackit-cli/internal/cmd/git/flavor/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/git/instance/list/list_test.go